To see for yourself how good this lens is have a look on my website in the 2004 section I've indicated which lens was in use. www.wildphotolife.co.uk
Sadly sigma are no longer making this prime, and now only produce the 300-800mm, so this review is only of use to those looking for second hand ones.
It's a big beast but as I've said its light for its focal length, at 4.74Kg its lighter than the 600mm canon and 600mm nikon. So a tripod is needed, but I have shot many flight shots handheld successfully, also have taken shots at low speeds balanced on walls and fence posts. It's just a matter of technique. With extension tubes its a cool, if somewhat excentric, macro lens. The bokeh is excellent.
I have sharp images with a 1.4 teleconvertor, I don't have a 2x so haven't tried that.
It's a bargain!!